Imagine a universe full of water. A comfortable 22 degrees Celsius warm, density of about 998 grams per liter, and it fills up everything for as far as can be observed. There's nothing else; neither empty pockets nor denser planets. Just water.
This would be an extremely massive universe, but if I understand the physics correctly there would be no spontaneous black hole formation. The density and temperature would be totally uniform, and so would the gravity: every H2O molecule would be pulled in every direction at once, with the resultant force being zero. No movement in the water, no build-up of mass, so no black holes.
What if some people from a different dimension pay a visit though... They venture through the Phlebotinum portal, find themselves in their spacecraft-turned-submersible in this weird universe, and be adding their denser-than-water selves to the mix. Suddenly water is pulled slightly in their direction, increasing local pressure (and local density, and thus local mass) even further... cascade into a black hole begins!
Or would it? Water doesn't like being compressed, so there is a high amount of force required to make water locally denser. That means there's a hurdle to overcome before the gravitationally compressed water becomes sufficiently dense (compared to standard water) to have sufficient gravity of its own to continue compression, and eventually collapse into a black hole.
I think it may just hold up against the addition of a single spacecraft, which would have minute gravity of its own. But I cannot tell for sure.
Can you quantify what local density variation would still be allowable in a water-filled universe, without collapsing the lot into a black hole? Could, for example, this universe have an Earth-sized rocky planet in it? Or would the addition of one grain of sand be enough to begin a cascade?
Assume all of our known laws of physics apply, except this universe is not expanding or contracting. I am primarily interested in the short-term effects of a new mass added to the water universe, not whether this universe would suffer a big crunch eons into the future.
How this strange universe came to be is out of the scope of the question :-) (short version: simulated universe)